Man’s unbelievable act of kindness

A random act of kindness has left a mother - and the rest of the world - in tears.

Kelsey Rae Zwick was travelling from Orlando to Philadelphia with her 11-month-old daughter, Lucy, for treatment at the Children's Hospital of Philadelphia when a complete stranger made a touching gesture.

In a Facebook post, Ms Zwick said she and Lucy had boarded American Airlines flight 588 when a flight attendant approached them and informed them a passenger in first class wanted to switch seats.

"To the man in 2D. Today you were travelling from Orlando to Philly" she started her post.

"I don't know you, but I imagine you saw us somewhere. I was pushing a stroller, had a diaper bag on my arm and also lugging an oxygen machine for my daughter. We had smiles on our faces as we were headed to see her 'friends' at CHOP (Children's Hospital of Philadelphia).

"We pre-boarded the plane, got cozy in our window seat and made jokes to those around us about having to sit by my yelling-but-happy baby. The flight attendant came over and told me you were waiting to switch seats. You were giving up your comfortable, first class seat to us."

Kelsey Rae Zwick with her twin daughters. Picture: Facebook

The touching gesture, which Ms Zwick said she "can't wait to tell Lucy someday", brought her to tears.

"Not able to hold back tears, I cried my way up the aisle while my daughter Lucy laughed!" she said.

Unable to thank the good samaritan properly as they passed each other in the aisle, Ms Zwick shared the story on Facebook in hopes to find the man. It quickly went viral and had many people were left in tears alongside Ms Zwick.

"To have been seen by this man and for him to extend himself in such a quiet, but significant way absolutely tugs every single one of my heart strings," wrote one person.

"Oh wow. That brought tears to my eyes knowing there ARE good people in the world," another wrote.

Lucy suffers from chronic lung disease but still has a smile on her face. Picture: Facebook

It doesn't appear Ms Zwick has found the mystery man yet but in the meantime she said she promises to "pay it forward".

"It reminded me how much good there is in this world.

"In the meantime … we will pay it forward. AA588 passenger in seat 2D, we truly feel inspired by your generosity."